Human cytomegalovirus (CMV) is a leading opportunistic viral pathogen, causing invasive disease such as retinitis, pneumonia, colitis and encephalitis in the immunocompromised. Successful management of AIDS and transplantation includes effective suppression of CMV infection, often involving prolonged antiviral therapy. Current therapy, based on ganciclovir, foscarnet and cidofovir, has a single viral target (DNA polymerase), and is complicated by dose-limiting toxicity, antiviral drug resistance and cross-resistance. Despite a strong clinical need and promising alternative antiviral drug targets, no new drugs have been FDA-approved in many years. The CMV UL97 kinase inhibitor maribavir is an important new treatment option because of oral bioavailability, a distinct viral target, and lack of cross-resistance with currently licensed drugs. After successful Phase I and II trials, ViroPharma conducted low-dose Phase III post-transplant prophylaxis trials which were unsuccessful but widely regarded as insufficiently dosed, because open label use of the drug at a higher dose appeared to salvage the treatment of several cases of refractory or drug-resistant CMV disease. During the past funding period, this research program identified viral UL97 and UL27 mutations that confer maribavir resistance, allowing for the timely genotypic diagnosis of the only maribavir-treated subject so far known to have developed resistance to this drug. In addition, the observed effects of cell culture conditions on in vitro maribavir susceptibility, and the synergistic effect of cellular antimetabolites, suggested that use of maribavir in combination with cellular kinase inhibitors may be beneficial. In the upcoming project period, research objectives pertaining to maribavir and other CMV antivirals are (1) use contemporary deep sequencing technology to track the evolution of drug resistance mutations, potentially enabling the earlier detection of impending resistance; (2) further develop phenotypic assays for CMV drug resistance by modifying control laboratory strains in genes UL128-131 to give them growth properties more similar to fresh clinical isolates, (3) evaluate the reported anti-CMV activity of cellular antimetabolites in clinical use for other indications, alone and in combination with existing antivirals; (4) assess the therapeutic potential of promising experimental compounds with defined CMV drug targets, with respect to potency, synergy, and propensity to resistance and cross-resistance. The expectation is that an ideally suppressive CMV therapy may involve a combination of drugs with different mechanisms of action, which could reduce the incidence of drug resistance, as in the treatment of other chronic viral infections.